Certain video applications, such as high-definition television (HDTV) require pixelated displays having array sizes as large as 1050.times.1700 or 1.8 megapixels. A bistable deformable mirror device (DMD) disclosed in the above-listed U.S. Pat. No. 5,061,049 entitled SPATIAL LIGHT MODULATOR AND METHOD, can be used as a projection light valve for such applications. For DMDs as large as 1.8 megapixels, it is extremely important to minimize the number of transistors per pixel in order to maximize chip yield and lower the cost.
The number of transistors required per pixel depends on whether the bistable DMD is operated unidirectional or bidirectional. In unidirectional operation, the torsion beam (reflective surface) is operated between its quiescent or flat state and a positive landing angle. The projection optics is designed so that the quiescent state is the dark state and the positive landing angle is the bright state.
The address sequence begins with bias being momentarily returned to ground, and the reflective beams reset with a reset pulse. The positive address electrode voltage, .theta..sub.a, is then set to either .theta..sub.a =+.vertline.V.vertline. or .theta..sub.a =0 by the address transistor, where V.sub.a is the value of the voltage placed on the address electrode and then the bias is turned back on. For +.vertline.V.sub.a .vertline. on the address electrode, the beam rotates to .theta.=+.theta..sub.L. For OV on where .theta. is the rotation angle of the beam, and .theta..sub.L is the angle of the beam when it lands the address electrode, the beam remains at .theta.=0.
In bidirectional operation, the torsion beam is operated between two landing states. The projection optics are designed so that one state is the dark state and the other state is the bright state.
Because the address circuit is more complex for bidirectional operation, requiring additional transistors, it would seem that unidirectional operation would be preferred. However, there are two limitations to unidirectional operation. First, a higher address voltage is required. Second, duty factor effects can lead to contrast degradation. These limitations are discussed in more detail in what follows.
In order to lower the address voltage requirement, a bias is applied to the beam. The amount of bias determines whether the beam is monostable, tristable, or bistable. For unidirectional operation, a potential energy barrier must be maintained between the flat state and the landed states. This barrier insures that the beam will remain in the flat state (for OV address) and not spontaneously deflect to either one of the landed states with application of the bias. Therefore, for unidirectional operation, the bias is limited to a level which insures a potential energy barrier adequate to prevent spontaneous deflection. This limitation on bias forces the address voltage to be increased. For example, a typical bistable DMD operated with no bias requires a 16 volt address. At a bias of -10V the DMD is operating in the tristable mode and requires a +10V address. At a bias of -16V the DMD is operating in the bistable mode and requires only a +5V address. It is clear in this example, that to be compatible with standard 5V CMOS address circuitry, it is necessary to operate in the bistable mode, which requires bidirectional operation and addressing.
Duty factor effects are a second limitation to unidirectional operation. When a torsion hinge is twisted, a portion of its surface is in compression and a portion in tension. Surface residues on the torsion hinges are subjected to these stresses. Over a sufficient period of time, these residues can stress relieve while in the twisted state. When the torsion hinge is then returned to its quiescent (untwisted) state, these residues provide a built-in stress that tends to keep the hinge twisted, and the beam is no longer flat in its quiescent state. The greater the deflection duty factor (i.e., the fractional length of time that the torsion hinge is in its twisted state) and the longer the time of operation, the greater the beam deflection angle when it is returned to its quiescent state.
This quiescent deflection is amplified by the differential bias and can amount to two to three degrees for a ten degree landing angle. Unless sufficient optical deadband is designed into the darkfield projection optics, this quiescent deflection can degrade the optical contrast.
This duty-factor stress relief mechanism is not avoided by going to bidirectional operation, but in that mode of operation, no contrast degradation is observed because the beam is operating between the two landing angles (.theta.=.+-..theta..sub.L) and not between the quiescent state (.theta.=0) and the positive landing angle (.theta.=+.theta..sub.L). Although the duty-factor effect has no influence on the contrast for the bidirectional mode of operation, the address voltage is influenced. A greater address voltage must be applied in one direction to compensate for the quiescent offset angle caused by stress relief. When a bidirectional DMD is quoted as operating at an address voltage of five volts, that five volts must include enough operating margin to allow for duty-factor offset.
Accordingly, a need exists in the art for a bistable deflection device which operates in the bidirectional mode in order to avoid the limitations of unidirectional operation, while at the same avoiding the more complex address circuit requirements of bidirectional operation.